.■^iCuu^  Eogineerioj 


UNIVERSITY 

OP 

fT.T,.IJBKAllY 


UNIVERSITY  OF  ILLINOIS 
LIBRARY 


Class  Book  Volume 

My  0S-15M  Ir 


(  OMPARATIVE  TESTS  OF  CARBO]Sr, 
M1J1A.LLIZED  CARBON  AND 
TANTALUM  FILAMENT 
LAMPS 

BY 

THOMAS   HAMER  AMRINE 


THESIS 

IN    PARTIAL   FULFILLMENT   OF   THE  REQUIREMENTS 

FOR  THE 

DEGREE  OF  ELECTRICAL  ENGmEER 


IN  THE 

GRADUATE  SCHOOL 

OF  THE 

UNIVERSITY  OF  ILLINOIS 


JUNE,  1908  X  , 


\r5  O'^ 


UNIVERSITY     OF  ILLINOIS 


May    26,          mo  8. 


THIS   IS   TO   CERTIFY   THAT  THE   THESIS   PREPAKED   UNDER  MY   SUPERVISION  BY 

 THOMA.S  ilAMEH  MRIM  

ENTITLED      Comparatlye    Testa    of    CarbQn,    Eetallized  CarbQa 
 Tantalum  Filament  .Lamps  

IS   APPROVED   BY  ME  AS   FULFIM^rXG  THIS  PART   OF  THE   REQUIREMENTS  l  OFv  THE  DEGREE 

 Electrioal  Engineer ,  


HEAD  OF  DEPARTMENT  OF    Eleotrioal  Engineering. 


university  of  illinois 
Engineering  Experiment  Station 


Bulletin  No.  19  September  1907 


COMPARATIVE  TESTS  OF  CARBON,  METALLIZED 
CARBON  AND  TANTALUM  FILAMENT  LAMPS 

By  T.  H.  Amrine,  B.  S.,  First  Assistant,  Department  of  Electrical 
Engineering,  Engineering  Experiment  Station 

At  the  present  time  there  are  only  three  types  of  incandes- 
cent lamps  having  a  wide  enough  commei'cial  use  to  make  them  im- 
portant factors  in  incandescent  lighting.  The  first,  and  by  far 
the  most  widely  used,  is  the  familiar  carbon  filament  lamp,  which 
in  ordinary  sizes  gives  an  efficiency  seldom  exceeding  3. 1  watts 
per  candle  power  with  an  elfective  life  of  approximately  500  hours. 
The  second  type  is  also  a  carbon  filament  lamp,  but  the  carbon  by 
the  process  through  which  it  passes  in  manufacture  is  given  some- 
what the  characteristics  of  a  metal,  and  for  this  reason  is  called 
the  metallized  filament  lamp.  The  manufacturers  have  claimed 
for  it  an  efficiency  of  about  2.5  watts  per  mean  horizontal  candle 
power.  In  the  third  type  the  filament  is  made  of  the  metal  tanta- 
lum and  there  is  claimed  for  it  an  efficiency  of  about  2  watts  per 
candle. 

It  is  the  purpose  of  this  bulletin  to  give  the  results  of  tests 
made  upon  these  lamps  in  the  laboratory  of  the  Electrical  En- 
gineering department  of  the  University  of  Illinois,  with  the  view  of 
bringing  out,  if  possible,  the  good  points  of  each  lamp  together 
with  any  other  facts  that  will  help  in  the  selection  of  the  proper 
type  for  any   particular  purpose.    Especial   care   was  taken 

The  writer  wishes  to  acknowledge  his  indebtedness  to  J.  M.  Bryant.  Associate  in  Electric- 
al Engineering,  for  valuable  sugtrestions  and  co-operation;  also,  for  aid  in  making  the  tests,  to 
R.  T-  Calloway.  L.  G.  Schumacher.  O.  M,  Ward  and  W.  R,  Scott,  of  the  Class  of  1907  in  Elec- 
trical Engineering. 


Digitized  by  the  Internet  Archive 

in  2013 


http://archive.org/details/comparativetestsOOamri 


2 


ILLINOIS  ENGINEERING  EXPERIMENT  STATION 


throughout  all  the  work  to  make  the  conditions  under  which  the 
tests  were  conducted  exactly  the  same  for  each  type  of  lamp  in 
order  to  have  a  fair  basis  of  comparison  between  types.  Although 
comparative  rather  than  absolute  results  have  been  particularly 
striven  for  it  is  felt  that  dependable  quantitative  values  have 
been  obtained. 

Description  of  Lamps* 

The  lamps  chosen  for  the  tests  were  selected  from  a  lot  of 
100  of  each  type  of  lamp.  These  were  bought  directly  from  the 
manufacturer,  a  well  known  and  reliable  incandescent  lamp  com- 
pany. 

Ratings. — The  carbon  lamp  was  rated  by  the  manufacturer  at 
25  candle  power  at  110  volts  with  an  efficiency  of  3.1  watts  per 
candle.  The  metallized  filament  lamps  were  rated  at  50  watts  at 
110  volts,  no  mention  being  made  of  candle  power  on  label.  The 
tantalum  lamps  had  a  rating  of  22  candle  power  at  110  volts. 

Filaments. — The  filaments  of  the  carbon  lamp  were  of  the  fa- 
miliar single-loop  type  anchored  in  the  middle  of  the  loop.  They 
had  a  smooth  uniform  appearance  and  were  of  the  steel-gray  color 
common  to  all  properly  flashed  carbon  filaments.  The  filaments 
of  the  metallized  lamp  were  not  so  smooth  in  appearance  as  those 
of  the  carbon  lamp.  They  had  a  kinked  appearance,  which,  ac- 
cording to  the  statement  of  the  manufacturer,  was  due  to  the  meth- 
od of  treating  the  filament  and  in  no  way  affected  the  efficiency  of 
the  lamp.  The  color  was  steel  gray,  slightly  darker  than  the  car- 
bon filament.  The  filaments  were  double,  consisting  of  two  horse- 
shoe loops,  the  inside  ends  of  which  were  attached  to  a  common 
anchor.  The  tantalum  filaments  were  very  fine  and  long,  as  was 
necessitated  by  the  rather  low  specific  resistance  of  the  metal  tan- 
talum. They  were  mounted  in  the  well  known  zigzag  fashion  upon 
supporting  spires.  The  color  was  a  silver-gray  with  a  metallic 
luster.  Fig.  1  is  a  photograph  of  the  three  types  of  lamps  used 
in  the  tests.  Table  1  gives  the  approximate  effective  dimensions 
of  the  three  types  of  filaments. 

*  At  the  time  these  tests  were  started,  tungsten  lamps  could  not  be  purchased  on  the  mar- 
ket- In  the  matter  of  current  consumption.life  and  candle  power  maintenance  these  lamps  seem 
much  superior  to  the  three  kinds  tested.  The  manufacturers  claim  an  efficiency  of  1  watt  per 
candle  with  a  life  of  1000  hours  with  a  decrease  in  candle  power  of  only  10  per  cent.  However, 
as  is  the  case  with  the  tantalum  lamps,  they  are  rather  delicate  and  require  even  more  care- 
ful handling. 


AMRINE — COMPARATIVE  TESTS  OF  LAMPS  3 


Carbon  Metallized  Tantalum 

Fig.  1 

TABLE  1 


Filament 

Effective  Length 
inches 

Mean  Diameter 

Effective  Area 
square  inches 

Carbon 

9.4 

.0060 

.1774 

Metallized 

9.5 

.0037 

.1108 

Tantalum 

23.4 

.0018 

.1324 

Bulbs. — The  shapes  of  the  bulbs  and  their  comparative  sizes 
are  shown  in  Fig.  1.  Those  of  the  carbon  and  metallized  lamps 
al-e  of  the  same  size  and  shape,  having  a  length  over  all  of  5  in., 
a  maximum  diameter  of  2^  in.  and  a  minimum  diameter  of  li  in. 
The  tantalum  lamp  is  about  5i  in.  in  length  and  has  a  maximum 
diameter  of  2H  in.  tapering  to  a  minimum  diameter  of  li|  in., 
thus  making  the  sides  much  more  nearly  parallel  than  in  the 
case  of  the  carbon  and  rcetallized  lamps. 


4 


ILLINOIS  ENGINEERING  EXPERIMENT  STATION 


Electrical  Characteristics 

The  three  types  of  lamps  differ  radically  in  their  temperature 
characteristics.  The  carbon  filament  has  a  negative  temperature 
coefficient;  that  is,  its  resistance  decreases  as  the  temperature  in- 
creases. On  the  other  hand,  on  account  of  the  treatment  which 
it  has  undergone  in  manufacture,  the  metallized  carbon  filament 
has  a  positive  temperature  coefficient  similar  to  the  metals  when 
in  the  incandescent  stage.  The  tantalum  filament,  being  of  met- 
al, has,  of  course,  a  positive  coefficient.  Fig.  2  plainly  shows  the 
increase  of  resistance  with  the  increase  of  temperature  in  the  met- 
allized and  tantalum  filaments  and  the  opposite  effect  in  the  car- 
bon after  reaching  the  incandescent  stage.  At  lower  temperatures 
the  change  is  probably  greater  than  after  it  becomes  incandes- 
cent, especially  in  the  carbon  lamp. 


O 


380 
360 
340 
320 
300 
280 
260 
240 


220 
200 
IHO 
160 
110 

120 


AN! 

"me 

TAl 

:ed 

■  CA 

.RB( 

1300       1400        1500        1600        1700        1800  1900 
Fig.  2   Filament  Temperature,  Degrees  Cent. 

As  a  result  of  the  positive  coefficients  of  the  metallized  and 
tantalum  filaments,  the  lamps  flash  up  to  full  incandescence  much 


AMRINE — COMPARATIVE  TESTS  OF  LAMPS 


5 


more  quickly  than  the  carbon  lamp.  When  the  current  is  turned 
on,  the  filament,  being  cold,  has  a  low  resistance  and  there  is  a 
rush  of  current  considerably  above  normal.  This  excessive  cur- 
rent is  rapidly  cut  down  as  the  lamp  reaches  incandescence  on 
account  of  the  increase  in  resistance.  The  carbon  lamp,  having 
the  greatest  resistance  when  cold,  allows  but  a  comparatively 
small  current  to  pass  at  first,  but  gradually  allows  it  to  increase 
as  the  resistance  becomes  less.  This  is  beautifully  shown  by  the 
oscillograms  of  the  rise  in  current  in  the  three  lamps  shown  in 
Fig.  3.    With  the  carbon  lamp  it  is  seen  that  the  current  almost 


s 


04      .06       .08      .10      .12      .14  .16 

Time  in  Seconds 


.20 


.26 


1.0 

.9 


/1E1 

ED 

-  r 

■)Rf 

c 

?CA 

r— 

1 

,02      .  04      .  06       .  08      .10       12       .14  .16 

Time  in  Seconds 


26 


1.0 
.9 


TAr 

LU 

M 

OR, 

c 

:  N 

T  - 

.0  .02 


.18 


20 


22 


04      .06      .08       ,1(1      .12      .14  .16 

Time  in  Seconds 
Fig.  .3   Oscillograms  op  Initial  Currents 

instantly  rises  to  about  .35  ampere,  and  then  rises  in  almost  a 
straight  line  to  the  full  steady  value  of  current  in  about  .26  sec- 


I 


« 


I 


6 


ILLINOIS  ENGINEERING  EXPERIMENT  STATION 


ond.  With  the  metallized  lamp  the  current  rises  at  once  to  about 
.45  ampere,  almost  the  full  steady  current,  then  increases  to  a  max- 
imum value  of  .55  ampere  in  approximately  .05  second,  indicating 
a  negative  temperatui-e  coefficient  at  the  lower  temperatures.  It 
then  decreases  gradually  to  the  normal  steady  value  in  about  .16 
second.  The  curve  for  the  tantalum  lamp  indicates  that  a  rush 
of  current  takes  place  as  soon  as  the  circuit  is  closed,  reaching 
a  maximum  of  about  .93  ampere,  almost  three  times  the  full 
steady  current,  practically  instantaneously.  This  rush  of  current 
in  the  tantalum  lamp  will  probably  require  that  some  precautions 
be  observed  in  switching  feeders  heavily  loaded  with  tantalum 
lamps  onto  the  generator.  The  heavy  excess  current  at  the  first 
instant  might  easily  be  sufficient  to  damage  the  machine.  The 
current  then  rapidly  drops  to  the  normal  value  which  is 
reached  in  about  .14  second.  A  suggestion  of  these  diffei'ences 
of  action  of  the  three  lamps  can  be  noticed  when  they  are  simul- 
taneously lighted  side  by  side.  The  carbon  lamp  appears  to  come 
at  once  to  a  rather  dull  incandescence  and  then  gradually  increases 
to  its  maximum  brilliancy.  The  metallized  lamp  appears  to  reach 
at  once  its  normal  incandescence  without  any  later  change  while 
the  tantalum  lamp  bursts  forth  immediately  in  brilliant  incandes- 
cence and  then  subsides  gradually,  giving  the  effect  of  a  flash  at 
the  first  instant. 

The  candle  power  voltage  characteristic  curves  in  Pig.  4  also 
show  important  differences  in  the  three  types  of  filaments.  At  80 
volts,  the  carbon  filament  starts  at  the  lowest  value  for  the  three 
lamps,  and  rises  rapidly  until  at  normal  voltage  and  above,  it  has 
the  greatest  candle  power.  The  tantalum  filament  takes  the  high- 
est position  at  the  80- volt  point  and  increases  more  gradually  un- 
til at  normal  voltage  and  above,  it  has  the  lowest  candle  power. 
The  metallized  filament  curve  takes  an  intermediate  position.  The 
equations  for  these  curves  obtained  by  the  method  of  least  squares 
from  the  experimental  data  are: 

For  the  carbon  lamp 

CP  =  143.5  X  10'''  X 

For  the  metallized  lamp 

CP  =  50.7  X  10""  X  E^-^ 
For  the  tantalum  lamp 

CP  =  166.4  X  10"'"  X  E'-''' 


AMRINE — COMPARATIVE  TESTS  OF  LAMPS 


7 


Below  is  shown  a  table  exhibiting  the  change  in  candle  power  for 
an  increase  of  5  per  cent  in  the  voltage  and  for  a  decrease  of  5  per 
cent  in  voltage  from  the  normal. 

TABLE  2 


Lamp 

C.  P.  Increase  for  Five  per 
cent  Increase  in  Voltage 
above  Normal  ' 

C.  P.  Decrease  for  Five  per 
cent  Decrease  in  Voltage 
below  Normal 

Carbon 

7.3   or   33.2   per  cent 

6.8   or  31.0  per  cent 

Metallized 

5.6   or   25.7   per  cent 

5.8   or   27.6   per  cent 

Tantalum 

4.4   or   22.0   per  cent 

4.8   or   24.0   per  cent 

80    85    90    95    100    105  110  115  120  125  130  135  140  145 
Volts 

Fig.  4  Characteristic  Curves  of  Candle  Power  and  Watts 

PER  Candle 


J 


8 


ILLINOIS  ENGINEERING  EXPERIMENT  STATION 


The  change  in  efficiency  or  watts  per  candle  is  also  shown  in 
Fig.  4,  indicating  a  wide  difference  in  favor  of  the  tantalum  lamp 
throughout  the  range  of  voltage.  The  curves  showing  the 
change  of  resistance  with  the  voltage  (Fig.  5)  indicate  that  the 


320 

300 
280 
260 

a  240 
o 


220 


2  200 


180 
160 
140 
120 


T 


-7- 


TANTALUM - 


METALLIZED: 


CARBON - 


10  20  30  40   50  60   70   80  90  100  110  120  130  140  150  160 
Volts 

Fig.  5  Curves  Showing  Change  of  Resistance  With  Voltage 

tantalum  and  the  metallized  filaments  tend  to  regulate  for  con- 
stant current.  In  these  filaments  the  resistance  rises  with  the 
voltage.  Hence  in  a  poorly  regulated  circuit,  when  there  is  an 
increase  in  pressure,  the  resistance  of  the  filament  becomes  great- 
er on  account  of  the  increase  in  temperature.  This  prevents 
such  a  great  rise  in  current  and  candle  power.  When  the  pres- 
sure drops  the  resistance  is  decreased,  thus  preventing  such  a 
large  decrease  in  current  and  candle  power.  With  the  carbon 
lamp  the  change  in  resistance  is  such  that  it  tends  to  aggravate 
the  effects  of  a  fluctuating  voltage.  When  the  voltage  increases 
there  is  a  decrease  in  resistance.  This  decrease  in  resistance 
adds  to  the  change  in  current  naturally  brought  about  by  the  in- 
crease in  pressure  and  the  result  is  a  very  rapid  change  in  cur- 
rent and  candle  power.  For  a  decrease  in  voltage,  of  course,  the 
reverse  is  true;  a  drop  in  voltage  causing  a  rise  in  resistance, 
the  change  in  both  of  them  being  in  the  direction  to  decrease  the 
curi'ent  and  candle  power. 


AMRINE- 


; — COMPARATIVE  TESTS  OF  LAMPS 


9 


Distribution 

Horizontal  and  vertical  distribution  curves  for  the  different 
lamps  when  new  and  after  800  hours  of  burning  are  shown  in  Fig.  6 
to  14.  The  small  figure  below  each  set  of  curves  indicates  the 
position  of  the  filament  in  each  case.  The  horizontal  distribution 
in  all  three  lamps  evidently  changes  equally  in  all  directions  after 
a  period  of  burning.  That  for  the  tantalum  lamp  would  be  almost 
a  circle  except  for  the  influence  of  the  leading-in  wires.  These 
cause  a  minimum  point  on  the  side  nearest  to  them.  The  vertical 
distribution  of  the  carbon  and  metallized  lamps  changes  but  little 
after  800  hours  of  service.  That  of  the  tantalum,  however, 
changes  to  a  marked  extent;  the  candle  power  for  the  position 
30°  from  the  tip  of  the  lamp  being  greater  after  burning  the  800 
hours  than  when  new.  The  change  in  the  spherical  reduction  fac- 
tor (the  constant  for  changing  mean  horizontal  candle  power  to 
mean  spherical  candle  power)  of  the  lamps  during  their  life  gives 
a  good  indication  of  the  way  the  distribution  changes.  Following 
is  a  table  of  these  values  for  three  periods  of  life. 

TABLE  3 


Lamp 

Spherical  Reduction  Factors 

New 

400  Hours 

800  Hours 

Carbon 

.810 

.805 

.794 

Metallized 

.803 

.805 

.801 

Tantalum 

.787 

.811 

.865 

The  causes  of  this  change  in  the  distribution  of  the  intensity 
in  the  tantalum  lamp  must  be  due  principally  to  the  change  in  the 
character  of  the  surface  of  the  filament  after  burning.  The  micro- 
photographs  of  the  filaments  shown  in  Fig.  30  indicate  how  rough- 
ened and  pitted  they  become  after  burning  for  a  few  hundred 
hours.  The  in-egularities  of  the  surface  cause  the  light  to  be 
radiated  and  reflected  from  their  surfaces  more  and  more  in  a  di- 
rection parallel  with  the  length  of  the  filament  as  the  period  of 
bui'ning  increases.    This,  of  course,  shifts  the  maximum  of  in- 


ILLINOIS  ENGINEERING  EXPERIMENT  STATION 


Fig.  6  Horizontal  Distribution  of  Carbon  Lamp  When  New 
AND  After  800  Hours  op  Burning 


AMRINE — COMPARATIVE  TESTS  OF  LAMPS 


11 


270° 

Fig.  7  Horizontal  Distribution  of  Metallized  Lamp  When 
New  and  after  800  Hours  of  Burning 


Fig.  8    Horizontal  Distribution  of  Tantalum  Lamp  When 
New  and  After  800  Hours  of  Burning 


I 


14  ILLINOIS  ENGINEERING  EXPERIMENT  STATION 


Fig.  10   Vertical  Distribution  at  90°  Azimuth  of  Metallized 
Lamp  When  New  and  After  800  Hours  of  Burning 


Fig.  11   Vertical  Distkibution  at  90°  Azimuth  of  Tantalum 
Lamp  When  New  and  Aftei{  800  Hours  of  Burning 


ILLINOIS  ENGINEERING  EXPERIMENT  STATION 


Fig.  12  Verttcal  Distribution  at  0°  Azimuth  op  Carbon  Lamp 
When  New  and  After  800  Hours  op  Burning 


ILLINOIS  ENGINEERING  EXPERIMENT  STATION 


Fig.  14   Vertical  Distribution  at  0°  Azimuth  of  Tantalum 
Lamp  When  New  and  After  800  Hours  op  Burning 


AMRINE — COMPARATIVE  TESTS  OF  LAMPS  19 

tensity  of  the  vertical  distribution  curves  further  from  the  hori- 
zontal. In  the  carbon  and  metallized  filaments  there  is  but  little 
change  in  the  character  of  the  surface,  and  consequently  the  dis- 
tribution in  these  lamps  changes  but  little  from  this  cause.  The 
manner  in  which  the  bulb  of  the  tantalum  lamp  discolors  after  use 
will  also  partly  explain  the  cause  of  the  change  in  distribution. 
This  discoloration  takes  the  form  of  a  band  of  black  deposit  on 
the  glass,  equal  in  width  to  the  distance  between  the  top  and  bot- 
tom supporting  spires.  Outside  of  this  band  there  is  a  deposit, 
but  it  is  much  lighter.  It  is  as  if  the  particles  of  the  metal  were 
projected  from  the  incandescent  filament  only  in  directions  normal 
to  its  surface.  The  density  of  this  deposit  in  the  band  cuts  down 
the  horizontal  intensity  a  great  deal,  but  at  the  tip,  where  the  de- 
posit is  thinner,  the  candle  power  is  decreased  by  a  much  less 
amount. 

Life  Tests 

Life  tests  were  made  of  the  three  kinds  of  lamps  under  two 
different  conditions.  Ten  lamps  of  each  kind  were  put  through 
the  life  and  efficiency  test  upon  a  steady,  well-regulated  voltage 
supplied  by  a  storage  battery.  The  battery  was  kept  floating 
across  nearly  constant  voltage  mains  and  a  large  rheostat  was 
put  in  series  with  the  lamps  with  which  to  make  the  finer  adjust- 
ments by  hand.  The  maximum  variation  was  probably  not  more 
than  one  volt  and  the  greater  portion  of  the  time  the  voltage  was 
as  nearly  correct  as  the  portable  voltmeter  used  would  indicate. 
This  was  designated  "Condition  A"  and  represents  the  best  con- 
ditions under  which  the  lamps  would  ever  be  operated  in  practice. 
The  same  number  of  lamps  were  operated  under  adverse  condi- 
tions. A  badly  fluctuating  alternating  current  was  supplied  to 
the  lamps  and  there  was  considerable  vibration.  This  condition, 
designated  "Condition  B",  is  representative  of  very  bad  operating 
conditions.  Pig.  15  to  20  show  the  candle  power  performance  of 
each  lamp  in  the  test.  The  uniformity  of  the  lamps  under  con- 
dition A  is  noticeable,  none  of  the  curves  varying  greatly  fi-om 
the  mean. 

Burn-outx  and  Faihires. — In  the  800  hours  of  the  test  under 
condition  A  only  three  lamps  were  lost.  Of  the  two  tantalum 
lamps  lost,  one  failed  by  the  breaking  of  the  glass  stem  and  the 
leading-in  wire,  probably  due  to  the  expansion  of  the  latter.  The 
other  failed  in  30  hours  probably  on  account  of  a  fault  in  the  fila- 


20  ILLINOIS  ENGINEERING  EXPERIMENT  STATION 

ment.  It  was  repaired  and  it  then  burned  at  a  high  candle  power 
for  a  short  time  and  then  burned  out.  The  failure  of  metallized 
filament  lamp  No.  2  was  due  to  the  fact  that  the  filaments  be- 


Candle  Power 


AMRINE — COMPARATIVE  TESTS  OF  LAMPS 


21 


22 


ILLINOIS  ENGINEERING  EXPERIMENT  STATION 


24 


ILLINOIS  ENGINEERING  EXPERIMENT  STATION 


26  ILLINOIS  ENGINEERING  EXPERIMENT  STATION 

came  ci'ossed  in  placing  the  lamp  in  the  socket.  Naturally  it 
burned  at  a  very  high  brilliancy  until  the  lamp  could  be  removed 
from  the  socket  and  the  cross  shaken  out.  When  this  was  done 
and  the  lamp  returned  to  its  place,  it  burned  out  in  a  very  short 
time  due  to  the  weakening  of  the  filament  while  at  the  high  tem- 
perature. The  operation  under  condition  B  shows  less  uniformity 
for  lamps  of  any  one  kind  and  a  great  increase  in  failures  during 
the  800  hours  of  the  test.  Only  one  carbon  lamp  burned  out  be- 
fore the  test  was  ended,  however.  Five  of  the  tantalum  and  eight 
of  the  metallized  lamps  failed  during  the  800  hours  of  burning  un- 
der this  condition,  thus  showing  the  great  superiority  of  the  old 
style  carbon  lamp  on  poorly  regulated  circuits  as  far  as  reliability 
is  concerned.  The  first  two  tantalum  and  the  first  two  metallized 
lamps  that  failed  did  so  early  in  their  life  and  the  failure  was 
probably  due  to  mechanical  defects  in  the  filament.  The  failures 
due  to  natural  causes  commence  then  at  about  400  hours  in  the 
metallized  and  at  about  550  hours  in  the  tantalum  under  these  con- 
ditions. The  poor  showing  of  the  metallized  filament  is  striking, 
it  being  much  poorer  than  the  tantalum,  which  is  not  claimed  to 
give  good  life  on  alternating  current  circuits.  The  combination 
of  poor  regulation  and  vibration  seems  to  be  very  detrimental  to 
its  long  life.  Possibly  the  frequency  with  which  the  lamps  had 
to  be  handled  had  something  to  do  with  the  large  number  of  burn- 
outs although  great  care  was  taken  throughout  the  tests  not  to 
subject  the  lamps  to  shocks  or  jars. 

Candle  Power  Maintenance  and  Change  of  Efficiency 

The  curves  in  Fig.  21  and  22  show  the  relative  changes  of 
candle  power  and  efficiency  for  the  three  kinds  of  lamps,  the 
curves  representing  the  mean  performance  of  the  lamps  tested. 
The  carbon  lamp  under  condition  A  stai'ts  out  with  a  high  candle 
power,  increases  comparatively  rapidly  for  the  fli'st  50  hours  or  so 
then  decreases  steadily  for  the  remainder  of  the  test.  The  tanta- 
lum lamp  rises  very  rapidly  for  the  first  20  hours  of  the  test,  then 
more  slowly  until  the  end  of  the  first  100  hours,  after  which  the 
candle  power  decreases  more  slowly,  after  400  hours  its  candle 
power  being  greater  than  that  of  the  carbon.  The  metallized 
lamp  changes  less  than  either  of  the  others.  It  rises  during  the 
early  period  of  burning,  remains  almost  constant  for  a  time, 
decreasing  slowly  in  candle  power  during  the  remainder  of  its  life. 


AMRINE— COMPARATIVE  TESTS  OF  T.AMPS 


1 


28  ILLINOIS  ENGINEERING  EXPERIMENT  STATION 

The  metallized  lamp  changes  no  more  in  efficiency  than  it 
does  in  candle  power.  The  carbon  lamp  changes  rapidly,  becom- 
ing less  and  less  efficient  with  respect  to  the  metallized  filament. 
The  tantalum  lamp  becomes  relatively  more  and  more  efficient 
than  the  metallized  lamp  during  the  first  250  hours  and  then  tends 


1  [ 


000;0-t<'MOOO'^-t'I^OCO?D-*MOOO?D'+iMOOOO-*<'M 


Watts  Per  Candle  Power 

COOOCO'M'M'MeMNi— lr-^rH>-(r-l 

Candle  Power 


AMRINE— COMPARATIVE  TESTS  OF  LAMPS 


29 


to  drop  off  and  approach  it  in  efficiency.  When  operating  under 
condition  B  the  changes  in  candle  power  and  efficiency  are  much 
the  same  except  that  the  changes  occur  more  rapidly.  It  is  no- 
ticeable that  in  this  case  the  carbon  lamp  starts  out  with  the  high- 
est candle  power,  but  after  500  hours  of  burning  it  has  the  lowest. 
Its  decrease  in  efficiency  is  correspondingly  rapid.  Under  con- 
dition B  the  tantalum  lamp  no  longer  becomes  less  efficient  rela- 
tively to  the  metallized  lamp  as  the  period  of  burning  increases 
as  it  does  under  condition  A.  Its  efficiency  curve  tends  to  fall 
further  and  further  below  that  of  the  metallized  lamp. 

The  reason  why  the  tantalum  and  metallized  filament  lamps 
show  a  better  efficiency  than  the  carbon  is  of  considerable  inter- 
est. There  is  no  doubt  that  the  greater  amount  of  the  superior 
efficiency  of  the  newer  types  of  lamps  is  due  to  higher  filament 
temperatures.  As  is  well  known,  when  a  solid  body  is  heated,  at 
first  only  the  long,  low  frequency  heat  waves  appear,  then  the 
red  light  waves,  and  as  the  temperature  is  further  increased, 
wave  lengths  corresponding  to  the  other  colors  of  the  spectrum 
through  the  violet  and  ultraviolet  appear.  If  for  any  tempera- 
ture we  measure  by  means  of  the  bolometer  the  intensity  of  radia- 
tion at  points  throughout  the  visible  and  invisible  portions  of  the 
spectrum  and  plot  these  values  against  the  wave  lengths,  we  get 
a  curve  similar  to  curve  A  in  Fig.  23,  having  a  maximum  at  some 
point,  m.  The  visible,  that  is,  the  light-giving  portion  is  shown 
unshaded.  As  the  temperature  is  increased,  the  maximum  of  this 
curve  moves  toward  the  region  of  shorter  wave  lengths,  as  shown 
in  the  curve  B.  There  is,  however,  an  increase  in  the  length  of 
each  ordinate  so  that  the  curve  does  not  move  bodily  down  the 
spectrum  with  an  increase  in  temperature,  but  the  ordinates  of 
the  energy  curve  move  toward  the  shorter  wave  lengths  by  an 
amount  such  that  the  product  of  the  corresponding  abscissas  and 
the  temperature  remains  constant  for  each  ordinate.  It  is  seen 
then  that  the  visible  portion  of  the  spectrum  is  a  greater  propor- 
tion of  the  entire  spectrum  than  at  the  lower  temperature  and 
hence  a  better  light  efficiency  results.  If  we  remember  that  the 
velocity  with  which  the  molecules  of  a  body  are  moving  increases 
with  the  temperature,  then  we  can  in  a  general  way  see  why  it 
is  that  the  point  of  maximum  intensity  in  a  continuous  spectrum 
is  shifted  toward  the  violet  as  the  temperature  increases. 


30 


ILLINOIS  ENGINEERING  EXPERIMENT  STATION 


////// /v 

///y//)(/////////i^\. 

i 

Fig.  23   Wave  Length 


The  curves  between  temperature  and  candle  power  per  square 
inch  of  filament  area,  or  emissivity,  shown  in  Pig.  24,  indicate 
that  the  tantalum  filament  has  a  lower  and  the  metallized  a  high- 
er emissivity  than  the  carbon  filament.  Since  emissivity  is  vital- 
ly connected  with  the  light  efficiency  of  an  incandescent  body,  the 
difference  in  the  relative  positions  of  the  emissivity  curves  of  the 
metallized  and  tantalum  lamps  with  respect  to  the  carbon  seems 
to  indicate  that  a  part  of  the  better  efficiency  of  the  two  newer  fil- 
aments is  due  to  diiferent  causes. 

The  tantalum  filament,  having  a  lower  emissivity  than  the 
carbon  filament,  requires  less  energy  to  maintain  the  same  tem- 
perature than  the  carbon;  that  is,  the  lower  emissivity  of  the  tan- 
talum filament  gives  it  a  better  efficiency  than  the  carbon  at  the 
same  temperature.  This  is  in  addition  to  the  fact  that 
tantalum  has  a  greater  atomic  weight  and  a  higher  vapor 
tension  point  than  carbon,  thus  allowing  it  to  be  operated  at  a 
higher  temperature  with  the  consequent  better  efficiency. 

Since  the  metallized  filament  has  a  higher  emissivity  than 
the  carbon  filament  it  must  require  a  greater  input  of  energy  per 
square  inch  of  surface  to  maintain  a  given  temperature.  Experi- 
ment shows  that  the  input  of  the  metallized  lamp  is  about  460 
watts  and  the  carbon  about  410  watts  per  square  inch  of  filament 
area  at  a  temperature  1720°  C.  To  give,  as  it  does  even  at  equal 
temperatures,  a  better  watt  per  candle  efficiency  it  must  then  give 
off  a  greater  proportion  of  light  energy  to  heat  energy  than  the 
carbon  lamp.    Since  the  carbon  filament  is  approximately  though 


AMRINE — COMPARATIVE  TESTS  OF  LAMPS 


31 


only  approximately,  equivalent  to  the  theoretical  solid  black  body 
this  fact  seems  to  show  that  the  greater  efficiency  of  the  metallized 
filament  must  be  due,  at  least  in  part,  to  a  sort  of  selective  ra- 
diation. That  is,  it  radiates  either  a  greater  proportion  of  its 
energy  within  the  range  of  the  visible  spectrum  than  a  black 
body  or  a  smaller  proportion  in  the  invisible  range.  In  Fig.  25 
and  26  the  dotted  curves  show  the  radiation  from  a  solid  black 
body.  Fig.  25  shows  the  curve  for  a  body  having  at  the  same 
temperature  almost  the  same  radiation  outside  the  visible  spectrum 
but  a  much  greater  radiation  within,  while  in  Fig.  26  is  shown  the 
curve  for  a  body  having  practically  the  same  radiation  within  the 


32 


ILLINOIS  ENGINEERING  EXPERIMENT  STATION 


Fig.  25  Wave  Length 


Fig.  26   Wave  Length 


visible  portion  of  the  spectrum,  but  a  much  less  radiation  without 
than  in  the  case  of  a  black  body.  In  one  of  these  cases  the  metal- 
lized filament,  no  doubt,  falls.  In  both  cases,  however,  there  is 
an  increase  in  efficiency  due  to  selective  radiation. 

In  the  cur  V  es  between  watts  per  candle  and  filament  temper- 
ature, there  is  another  indication  that  the  high  efficiency  of  the 
metallized  and  tantalum  lamps  is  due  to  different  causes.  It  is 
seen  in  the  curves  of  Fig.  27  that  at  ordinary  efficiencies  the  tan- 
talum is  the  lowest,  the  metallized  next,  while  the  carbon  is  the 
highest.  However,  at  about  1830°  C. ,  the  curves  for  the  metal- 
lized and  tantalum  filaments  cross  and  hence  for  higher  tempera- 
tures the  metallized  filament  has  the  better  efficiency.    When  the 


AMRINE— COMPARATIVE  TESTS  OF  I^AMPS 


33 


2100 


2000 


1900 


1800 


1700 


1600 


r  1500 


1400 


4 

'  i\/ri 

RB( 

-  T  A 

)N 

T  I  I 

ZED 



•ANTALUM 

1      2     3      4      5      6      7      8      9     10     II     12  13 
Fig.  27   Watts  Per  Candle  Power 

curves  are  produced  to  about  1875°  C,  the  curve  for  the  tantalum 
filament  crosses  that  for  the  carbon  so  that  beyond  this  point  the 
latter  would  be  more  efftcient  if  it  could  be  operated  at  such  tem- 
peratures. This  too  shows  that  it  is  due  to  the  ability  of  the  tan- 
talum filament  to  withstand  high  temperatures  without  too  rapid 
disintegration  that  it  has  so  high  an  efficiency. 

When  we  consider  the  curve  for  the  metallized  filament  with 
respect  to  that  of  the  carbon  it  is  seen  that  it  falls  below  that  of 
the  latter  and  tends  to  fall  further  below  it  at  the  higher  temper- 
atures; that  is,  as  the  temperature  is  increased,  the  metallized 
filament  becomes  relatively  more  and  more  efftcient  for  any 


34 


IT.TJNOIS  ENGINEERING  EXPERIMENT  STATION 


given  temperature.  Evidently  then  it  is  due  not  so  much  to  its 
ability  to  withstand  high  temperatures  that  the  metallized  fila- 
ment is  the  more  efficient  but  rather  to  a  selective  radiation  that 
becomes  more  and  more  pronounced  as  the  temperature  is  in- 
creased. 

Connected  closely  with  the  life  of  the  lamps  is  the  condition 
of  the  filaments  after  a  period  of  burning.  The  carbon  filament 
is  shown  in  the  micro- photographs*,  Fig.  28,  when  new,  after  1000 


jirt>w»i<««i,iaii'ii|''i|*ti' " 


Fig.  28  Carbon  Filament 


hours'  burning  under  condition  A  and  after  800  hours'  burning 
under  condition  B.  Little  or  no  disintegration  or  breaking  up  of 
the  filament  is  shown.  It  is  almost  as  smooth  and  strong  looking 
after  800  hours  of  the  hard  service  as  it  was  when  it  was  new. 
Fig.  29  shows  the  micro- photographs  of  the  metallized  filaments 
when  new,  after  1000  hours  under  condition  A  and  800  hours  un- 
der condition  B.    It  is  seen  that  after  the  different  periods  of 


Fig.  29    Metallized  Filament 


♦Taken  by  Mr.  David  Klein,  Department  of  Chemistry  of  University  of  Illinois. 


AMRINE— COMPARATIVE  TESTS  OF  LAMPS 


35 


burning,  the  filament  is  not  quite  as  smooth  as  when  new,  but  is 
pitted  somewhat  and  has  decreased  in  size  a  little.  The  most  re- 
markable change,  however,  is  shown  in  the  case  of  the  tantalum 
filament  in  Fig.  30.    When  new  it  is  smooth  and  cylindrical,  show- 


FiG.  30    Tantalum  Filament 


ing  slight  pittings  or  markings.  After  1000  hours  under  condi- 
tion A  it  has  roughened  up  a  great  deal, being  covered  with  notches 
and  ridges  due  perhaps  to  unequal  evaporation  of  the  filament. 
The  filament  burned  on  alternating  current  shows  a  greater 
change  even  than  that  burned  under  condition  A.  It  appears  to 
have  a  sort  of  a  segmented  structure;  in  fact,  in  places  along  the 
filament  it  appears  as  if  small  sections  had  fallen  out  a  part  of  the 
way  and  had  been  caught  and  welded  again.  When  the  filament 
is  in  this  condition  even  a  slight  jar  will  serve  to  shatter  the  en- 
tire filament.  One  lamp  after  burning  for  almost  800  hours 
under  condition  B  was  dropped  a  short  distance.  After  being 
lighted  it  was  found  that  the  filament  had  been  broken  and  weld- 
ed together  again  in  no  less  than  eight  places. 

A  summary  of  the  performance  of  the  lamps  on  the  life  and 
efficiency  tests  is  shown  in  Table  4,  together  with  a  table  of  the 
costs  of  energy  and  renewals  at  different  rates  per  kilowatt-hour. 
This  latter  table  is  shown  graphically  in  the  curves  of  Fig.  31  and 
32.  These  curves  are  plotted  between  "Total  cost  in  cents  per 
candle  power  hour  for  lamps  and  energy"  as  ordinates  and  "Cost 
of  energy  per  kilowatt-hour"  as  abscissas.  Fig.  31  shows  the 
curves  for  lamps  operating  under  condition  A,  that  is,  upon  a 
very  well  regulated  direct  current  circuit,  while  Fig.  32  is  for 


36 


ILLINOIS  ENGINEERING  EXPERIMENT  STATION 


TABLE  4 

Summary  of  Life  and  Efficiency  Tests 


Carbon 

Metallized 

Tantalum 

Operating  Condition 

A 

B 

A 

B 

A 

B 

Av.  Mean  Horizontal  C.  P. 

(a)  (new) 

(O)    (wv  nrs. ) 
(c)    (800  hrs.) 
Spherical  Reduction  Fac- 
tor (a)  (new) 

(b)  (400  hrs.) 

(c)  (800  hrs.) 

Av.  Spherical  Candle  Pow- 
er  (a)  (new) 

(b)  (400  hrs.) 

(c)  (800  hrs.) 
Av.  Watts  per  Lamp 

Av.  Initial  Watts  per  C.  P. 

24.9 
19.7 
16.5 

.810 
.803 
.794 

20.2 
15.8 
13.1 
73.3 
3.00 

25.2 
16.4 
12.3 

20.4 
13.2 

9.8 
72.6 

3.1 

20.6 
18.1 
14.9 

.803 
.805 
.801 

16.6 
14.6 
11.9 
51.9 
2.62 

20.5 
16.1 
14.2 

16.5 
13.0 
11.4 
51.8 
2.62 

19.8 
19.9 
17.2 

.797 
.811 
.865 

15.8 
16.1 
14.9 
42.0 
2.02 

19.8 
16.5 
14.8 

15.8 
13.4 
12.8 
39;7 
1.99 

Rated  Watts  per  C.  P. 

3 

.1 

2 

5 

2. 

0 

Total  Lamp  Hours 
Total  Candle  Power  Hours 
Total  Kilowatt  Hours 
No.  Burnouts  in  800  Hrs. 

8180 
165236 
599.6 
0 

7766 
135905 
563.8 
1 

7963 
144130 
413.3 
1 

4900 
80905 
253.8 
8 

6779 
137614 
284.7 
2 

6123 
105928 
243.1 
4 

Cost  of  Lamps  Each 

$ 

17 

$ 

25 

$. 

51 

Total  Cost  Lamps  &  Re- 
newals 

$1.70 

$1.87 

$2.75 

$4.08 

$6.12 

$6.83 

Cost  of  Power  Per  Kw. 
Hr. 

$  .01  f 
$  03 

$  .05       Total  Cost  1 
$  .07        of  Lamps  J 
$  .10      and  Energy  1 
$  .12      Per  CP. Hi-,  j 
$  .15 

$  .20  [ 

$  .00466 
$  .0119 
$  .0192 
$  .0264 
$  .0373 
$  .0447 
$  .0555 
$  .0736 

$  .00553 
$  .0138 
$  .0221 
$  .0305 
$  .0429 
$  .0512 
$  .0637 
$  .0843 

$  .00477 
$  .0105 
$  .0162 
$  .0220 
$  .0.306 
$  .0363 
$  .0449 
$  .0593 

$  .00818 
$  .0144 
$  .0207 
$  .0270 
$  .0.364 
$  .0427 
$  .0520 
$  .0678 

$  .00652 
$  .0107 
$  .0148 
$  .0189 
$  .0252 
$  .0293 
$  .0.356 
$  .0459 

$  .0092 
$  .0133 
$  .0179 
$  .0225 
$  .0295 
$  .0343 
$  .0418 
$  .0524 

AMRINE — COMPARATIVE  TESTS  OF  LAMPS 


37 


H  .01    .03   .05   .07   .09   .11    .13   .15  .17    .19   .21   .23  .25 


Fig.  31   Cost  of  P^nergy  Pek  K.  W.  Hour 

lamps  upon  pooi'ly  regulated  alternating  current  circuit  with 
some  vibration,  that  is,  condition  B. 

Considering  the  curve  for  the  carbon  lamp  working  under 
condition  A  it  is  seen  that  for  very  low  prices  per  kilowatt-hour 
for  power,  this  lamp  is  the  most  economical  on  account  of  the 
small  number  of  burn-outs  and  the  low  cost  of  the  lamps.  For 
costs  of  power  from  $.011  to  $.022  per  kilowatt-hour  the  metal- 
lized lamp  gives  the  lowest  cost,  while  for  all  the  higher  prices  of 
energy  the  tantalum  gives  the  best  economy.  For  condition  B 
the  relative  performance  does  not  change  a  great  deal,  though 
on  account  of  the  large  number  of  burn-outs  with  the  metallized 
lamp  at  no  time  does  it  give  the  most  economical  results. 

These  results  seem  to  show  that  so  far  as  economy  of  oper- 


38 


ILLINOIS  ENGINEERING  EXPERIMENT  STATION 


/"carbon 


Fig.  32  Cost  of  Energy  Per  K.  W.  Hour 

ation  goes,  the  metallized  lamp  has  practically  no  field  in  incan- 
descent lighting.  From  the  standpoint  of  low  cost  of  renewals, 
an  important  item  with  lighting  companies  that  furnish  free  re- 
newals, it  cannot  compete  with  the  carbon  or  tantalum  lamp, 
especially  upon  poorly  regulated  circuits  and  where  there  is  vi- 
bration or  rough  usage.  In  cost  of  power  consumption  the  car- 
bon lamp  leads  for  very  low  costs  of  power,  and  the  tantalum  for 
higher  costs  of  energy.  The  metallized  lamp  seems  to  have  a 
narrow  field  upon  very  well  regulated  circuits  where  the  cost 
is  between  $.02  and  $.03  per  kilowatt-hour. 

Summary 

As  a  summary  it  might  be  well  to  consider  separately  the 
three  lamps  and  compare  them  with  respect  to  the  following 


AMKINE — COMPARATIVE  TESTS  OF  LAMPS 


39 


eight  considerations  which  determine  the  choice  of  an  incandes- 
cent lamp. 

1.  Efficiency. 

2.  Cost  of  Operation. 

3.  Maintenance  of 'Candle  Power  and  Efficiency. 

4.  Life. 

5.  Quality  of  Light. 

6.  Distribution  of  Light. 

7.  Susceptibility  to  Voltage  Variations. 

8.  Ability  to  Withstand  Rough  Usage. 

1 .  Efficiency 

In  the  matter  of  efficiency  alone,  this  test  as  well  as  all  other 
tests  which  have  been  made  with  these  lamps  shows  conclusively 
that  the  metallized  lamp  is  much  superior  to  the  carbon,  and  the 
tantalum  is  as  much  superior  to  the  metallized.  The  difference 
between  3.1  watts  per  candle  and  2.0  watts  per  candle,  about  28 
per  cent,  is  sufficient  to  outweigh  almost  all  other  considerations. 
It  means  that  a  20  candle  power  metallized  or  22.5  candle  power 
tantalum  lamp  can  be  operated  with  the  same  amount  of  energy 
as  a  16  candle  power  carbon  lamp.  It  means  that  a  power  plant 
which  is  running  with  a  heavy  overload  of  carbon  lamps  would,  if 
the  carbon  lamps  were  exchanged  for  the  same  number  of  candle 
power  of  the  newer  lamps,  be  operating  at  about  normal  load 
with  the  consequent  advantages.  In  the  same  way  a  method  is 
provided  to  lighten  overloaded  feeders  without  any  decrease  in  the 
candle  power  of  light  furnished. 

2.  Cost  of  operation 

The  curves  of  Pig.  31  and  32  show  that  upon  well  regulated 
circuits  each  type  has  a  field  of  its  own  within  which  it  is  the  most 
economical.  For  [very  low  costs  of  power  the  carbon  lamp  gives 
the  best  economy.  Hence  particularly  for  persons  who  generate 
their  own  current  it  would  not  pay  to  change  from  carbon  to  the 
higher  efficiency  lamps,  because  in  this  case  either  the  cost  of 
power  is  low  or  else  the  fuel  bill,  the  only  item  in  which  there 
would  be  a  saving  by  using  high  efficiency  lights,  is  not  large 
compared  with  the  other  expenses  such  as  attendance  charges, 
taxes  and  interest.  When  the  cost  of  energy  is  high,  as  it  is  in 
most  cities,  the  tantalum  lamp  would  be  the  best  to  use.  The  metal- 
lized lamp  seems  to  be  restricted  to  rather  narrow  limits  of  power 


40  ILTJNOIS  ENGINEERING  EXPERIMENT  STATION 

cost  and  to  good  operating  conditions.  The  newer  types  of 
lamps  would  have  a  great  field  in  lighting  railroad  trains  and 
steamships  where  the  cost  of  power  is  always  high,  if  filaments 
were  robust  enough  to  withstand  the  shocks  and  vibrations  that 
are  usually  present.  It  seems  that  it  might  be  possible  and 
advisable  for  manufacturers  to  develop  series  tantalum 
lamps  for  this  purpose.  The  filaments  that  would  be  used 
in  a  series  lamp  would  no  doubt  be  strong  enough  to  withstand 
the  vibrations  and  jars  found  in  this  service  and  their  economy  of 
current  consumption  would  make  them  much  preferable  to  the 
carbon  lamp. 

3.  Maintenance  of  candle  power  and  efficiency 

In  regard  to  maintaining  candle  power  and  efficiency  the 
newer  lamps  make  by  far  the  best  showing,  the  two  being  almost 
the  same  in  this  respect.  The  metallized  and  tantalum  lamps 
have  a  drop  of  respectively  20  and  19  per  cent  in  candle  power  in 
1000  hours  under  condition  A  while  the  carbon  drops  82  per  cent 
in  the  same  time  and  under  the  same  conditions.  The  change  in 
efficiency  for  the  three  lamps  is  in  about  the  same  proportion. 

4.  Life 

Comparing  the  lamps  upon  the  basis  of  average  life  to  80  per 
cent  of  the  original  candle  power,  which  is  standard  for  the  car- 
bon lamp,  the  following  results  are  obtained. 


TABLE  5 


Condition 

Life  in  hours 

Carbon 

Metallized 

Tantalum 

A 

400 

780 

820 

B 

225 

350 

350 

This  method  of  comparison  is  if  any  thing  unfair  to  the  higher 
efficiency  lamps,  because,  owing  to  their  higher  first  cost,  the 
smashing  point  should  be  after  the  lamps  have  reached  a  candle 
power  considerably  less  than  80  per  cent  of  the  original.  It 


AMRINE — COMPARATIVE  TESTS  OF  LAMPS 


41 


serves,  however,  to  show  the  superiority  of  the  newer  lamps  in 
this  respect. 

5.  Quality  of  light 

The  quality  of  the  light  from  the  metallized  and  tantalum  lamps 
is  much  the  same,  both  being  considerably  whiter,  softer  and  more 
pleasing  to  the  eyes  than  that  of  the  carbon  lamp.  Being  a 
whiter  light,  the  newer  lamps  show  more  nearly  the  true  values 
of  colors  than  the  carbon  lamp  and  hence  are  superior  for  light- 
ing dry  goods  and  clothing  stores,  art  and  picture  galleries  and 
other  places  in  which  colors  must  be  judged.  The  intrinsic  bril- 
liancy of  the  three  kinds  of  filaments  is  approximately 

Carbon  140  C.  P.  per  sq.  in. 
Tantalum  165  C.  P.  per  sq.  in. 
Metallized  190  C.  P.  per  sq.  in. 

On  account  of  the  great  intrinsic  brightness  of  the  newer  types 
of  filaments,  particularly  the  metallized,  it  is  not  advisable  to  use 
these  lamps  for  interior  lighting  when  they  are  placed  low  enough 
to  be  in  the  line  of  vision,  unless  they  are  provided  with  ground 
glass  or  opal  globes. 

6.  Distribution  of  light 

The  distribution  of  the  carbon  and  metallized  lamps  is  so 
nearly  identical  as  to  admit  of  little  choice  between  them  in  this 
particular.  The  tantalum  lamp  differs  from  these  in  having  a  low 
tip  candle  power  which  is  a  point  in  its  favor  when  used  with  re- 
flectors. 

For  use  with  reflecting  globes  the  most  efficient  lamp  for  any 
given  watt  per  candle  consumption  would  be  one  with  a  long 
straight  filament  mounted  vertically.  This  kind  of  an  arrange- 
ment gives  the  condition  where  the  minimum  proportion  of  light 
is  radiated  downward  and  upward,  but  gives  a  distribution  which 
can  be  changed  to  suit  the  requirements  by  means  of  reflectors, 
and  is  such  that  the  minimum  light  is  lost  in  the  base.  Getting 
a  downward  distribution  by  placing  the  greater  part  of  the  fila- 
ment horizontal,  as  has  been  done  in  many  of  the  "downward 
light"  lamps  on  the  market,  is  an  inefficient  method.  It  is  true 
that  these  lamps  throw  the  maximum  of  their  intensity  further 
down  from  the  horizontal  than  an  ordinary  lamp,  but  in  so  doing 
just  as  much  light  is  thrown  upward  where  it  is  mostly  lost  in  the 


42  ILLINOIS  ENGINEERING  EXPERIMENT  STATION 

base  and  on  the  ceiling  by  absorption  and  improper  i-eflection. 
To  get  light  where  it  is  needed  and  do  it  most  efficiently  is  accom- 
plished by  mounting  the  filament  so  that  as  nearly  as  possible  the 
entire  length  of  it  is  parallel  to  the  axis  of  the  base,  and  then  us- 
ing good  reflectors.  The  tantalum  lamp  meets  this  requirement 
very  well,  as  is  shown  by  its  low  tip  candle  power  which  indicates 
a  small  loss  of  light  in  the  base.  The  intensity  at  angles  even  up 
to  30°  from  the  tip  is  low  in  this  lamp.  The  same  condition 
would  be  shown  at  the  base  if  the  distribution  were  not  changed 
by  its  presence.  Near  the  base  all  light  that  has  its  course  changed 
downward  by  reflectors  must  strike  the  reflector  at  a  large  angle. 
This,  of  course,  is  a  condition  that  favors  absorption  and  losses. 
With  the  tantalum  the  radiation  in  these  unfavorable  directions 
is  less  than  in  the  other  two  and  is  superior  for  that  reason  if  re- 
flectors are  used.  When  the  lamps  are  used  bare  the  cai'bon  and 
metallized  give  a  greater  candle  power  downward  where  it  is  gen- 
erally needed  than  does  the  tantalum  lamp. 

7.  Susceptibility  to  voltage  variation 

Table  2  gives  a  comparison  of  the  way  these  lamps  act  in  re- 
gard to  the  very  important  point  of  susceptibility  to  voltage  var- 
i  ation.  For  use  upon  poorly  regulated  feeders  or  at  the  end  of 
long  feeders  that  ai'e  sometimes  heavily  loaded,  the  metallized  and 
tantalum  lamps  will  make  a  much  more  uniform  and  pleasing 
light  than  the  sensitive  carbon  lamp. 

8.  Ability  to  withstand  rough  nsage 

It  is  in  this  particular  that  the  carbon  lamp  stands  supreme. 
Long  experience  in  making  them  has  enabled  the  manufacturers 
to  make  a  carbon  filament  that  will  withstand  almost  any  reason- 
able usage.  The  filaments  of  both  the  metallized  and  the  tanta- 
lum lamps  are  easily  broken,  especially  after  they  have  been 
burned  for  a  while.  The  filament  of  the  former  is  so  fine  that 
jars  such  as  would  be  caused  by  screwing  the  lamp  into  or  out  of 
the  socket  sometimes  make  the  two  halves  of  the  filament  cross 
eacli  other  near  the  top.  This  short-circuits  about  one-third  of 
the  filament,  and  if  the  current  is  turned  on,  the  lamp  then  burns 
at  about  three  times  the  normal  candle  power.  This,  of  course, 
greatly  reduces  the  life  of  the  lamps  if  the  filaments  are  not  sep- 
arated. They  may  be  shaken  apart  by  tapping  the  lamp,  but  usu- 
ally not  before  the  filament  has  been  materially  weakened  by  burn- 


j. 


AMRINE — COMPARATIVE  TESTS  OF  LAMPS  43 

ing  at  the  high  candle  power.  The  filament  of  the  metallized  lamp 
is  easily  set  to  vibrating  in  an  annoying  manner.  Often  while 
working  with  them  the  vibration  of  the  filaments  was  such  that 
the  flicker  was  easily  seen  on  a  piece  of  white  paper  at  a  distance 
of  four  or  five  feet  from  the  lamp.  It  was  only  in  some  positions 
about  the  lamp  that  this  was  noticeable,  but  in  these  positions  it 
was  very  pronounced  and  disagreeable.  It  seems  to  be  caused 
principally  by  the  reflections  from  the  bulb  of  the  lamp.  The 
motion  was  in  this  way  magnified.  No  such  effect  could  be  ob- 
tained from  the  carbon  or  tantalum  lamps.  The  carbon  filaments 
were  stiff  enough  to  resist  the  vibrations  and  the  way  in  which  the 
tantalum  filaments  were  mounted  prevented  any  vibration  in  them. 

Conclusion 

From  the  study  of  these  lamps  it  appears  that  the  carbon  fil- 
ament and  the  tantalum  filament  lamps  can  cover  adequately  all 
the  phases  of  incandescent  lighting  that  are  now  covered  by  the 
three  types.  For  low  power  costs  and  for  rough  or  unusual  uses 
and  for  small  candle  power  units  the  carbon  lamp  is  best  and 
often  the  only  one  that  can  be  used.  For  higher  costs  of  power 
upon  poorly  regulated  circuits  and  for  lightening  the  load  upon 
overloaded  stations  the  tantalum  lamp  is  best.  It  is  not  recom- 
mended by  its  manufacturers  for  use  upon  alternating  current,  yet 
the  results  obtained  show  that  although  it  does  not  do  so  well 
upon  alternating  current  as  it  does  upon  direct  current  circuits,  it 
still  gives  better  economy  for  the  higher  power  costs  than  the 
carbon  lamp.  The  principal  fault  of  the  metallized  lamp  is  that 
of  mechanical  weakness,  which  probably  does  not  exist  in  the 
larger  sizes  where  a  heavier  filament  is  used,  so  that  for  units  of 
40  or  60  candle  power  or  above,  this  type  of  lamps  is  very  satis- 
factory. 


4- 


4 


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